Limits Seen On Human Existence

By MALCOLM W. BROWNE

Published: June 1, 1993

IS it possible, using statistics alone, to predict how long space exploration, the Catholic Church, the Miss America contest, the human race or the universe as a whole will continue to exist? A Princeton University astrophysicist has concluded that these and many other predictions are not only possible, but can be made with 95 percent reliability.

As he described his hypothesis in the current issue of the British journal Nature, the scientist, Dr. J. Richard Gott 3d, proposed that the longevity of things can be estimated remarkably well from their histories. All that is needed to calculate roughly how long something is likely to last is to assume that there is nothing special about it -- nothing to particularly distinguish it from all the other objects or systems or creatures or intervals of time in its class. Theorists call this assumption the "Copernican principle," referring to the Polish astronomer Copernicus, who concluded that the Earth was not at the center of the solar system (or universe), and therefore did not occupy any "special" place.

Based on this seemingly simple idea, Dr. Gott predicted that intelligent human beings would probably exist on Earth a maximum of 7.8 million years, a minimum of 5,128 years, or somewhere in between. This calculation, he said, requires no knowledge of how or why the human race might end, and is based solely on the fact that Homo sapiens first appeared on Earth about 200,000 years ago.

The prediction, he said, is in good agreement with the life spans of many comparable species, including Homo erectus, an ancestor of modern man, which lasted about 1.8 million years before becoming extinct. Predictions of this kind are a statistical way of saying that the longer something has been around, the longer it is likely to survive in the future.

Given the probability that Homo sapiens has a life span of a few million years, of which 200,000 have already passed, Dr. Gott suggests making the best possible use of the time remaining. For one thing, he believes people should be preparing to seek habitats elsewhere in the galaxy while they still have a chance.

Knowing only that the manned space program has existed for 32 years, he says, one can calculate, using the Copernican principle, that the space program is 95 percent likely to continue for a maximum of 1,250 years, or a minimum of 10 months. If that is the case, people have very little time to begin colonizing the galaxy and establishing civilizations on other habitable planets before the manned space program ends, for whatever reason. Having lost the chance to colonize the galaxy, humans would be marooned on Earth until their inevitable extinction.

Dr. Gott considers it unlikely that humans -- or any other civilization -- will colonize a galaxy in 1,250 years or less. This, he says, could account for the fact that there is no evidence of colonization anywhere in the galaxy. His assumption is that there is nothing special about the life span of the Earth's manned space program. Therefore, he reasons, the manned space programs of any other civilizations would probably have about the same longevity as mankind's, likewise insufficient to colonize the galaxy.

"A lot of people say there's no need for a manned space program, because we can learn just as much by sending out interstellar robots," Dr. Gott said. "But this ignores the desire by the human race to survive. The Earth is a pretty dangerous place. If we hope to survive in the long term, it would make sense to start colonizing other habitable planets. We'll never do that unless we push forward with a manned space program, and our opportunity may not last very much longer."

Would emigrating to another planet set back the clock for the human race so that it could start anew and survive another couple of million years?

"That's a question we can't answer from this vantage point on the Earth today," Dr. Gott says, "because it hasn't happened yet, it has no past and, therefore, we cannot calculate its future."

The mathematics used in these calculations assumes that the life span of anything, or group of things, can be represented by 40 equal intervals of time, each representing 2.5 percent of the total life span. If the objects or creatures or systems encompassed by their respective life spans are randomly distributed, the chances are 39 to 1 against any individual being in the earliest 2.5 percent of the life span, and equally against being in the last 2.5 percent. How Long Will You Live?

"Looking at the time line of the human race," Dr. Gott said, "all you know is that you're located somewhere after its beginning and somewhere before the end.

"We're doing this calculation at the 95 percent confidence level, so that means that it's 95 percent likely that you're in the middle 95 percent of the life span of the human race, not in the first 2.5 percent or the last 2.5 percent. Your particular 2.5 percent is one-fortieth of the total longevity, so if you're at the beginning of that middle period, you have one-fortieth of the Homo sapiens life span behind you and 39-fortieths to go. But if you happen to be in the last 2.5 percent, you only have one-fortieth to go and the future is only one thirty-ninth as long as the past."